View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Access to Research and Communications Annals Int. J. Dev. Biol. 49: 615-632 (2005) doi: 10.1387/ijdb.052023lb Gametophyte interaction and sexual reproduction: how plants make a zygote LEONOR C. BOAVIDA1, ANA MARIA VIEIRA1, JÖRG D. BECKER1 and JOSÉ A. FEIJÓ*1,2 1Centro de Biologia do Desenvolvimento, Instituto Gulbenkian de Ciência, Oeiras, Portugal and 2Universidade de Lisboa, Faculdade de Ciências, Dept. Biologia Vegetal, Campo Grande, Lisboa, Portugal ABSTRACT The evolutionary success of higher plants relies on a very short gametophytic phase, which underlies the sexual reproduction cycle. Sexual plant reproduction takes place in special organs of the flower: pollen, the male gametophyte, is released from the anthers and then adheres, grows and interacts along various tissues of the female organs, collectively known as the pistil. Finally, it fertilizes the female gametophyte, the embryo sac. Pollen is released as bi or tricellular, highly de-hydrated and presumably containing all the biochemical components and transcripts to germinate. Upon hydration on the female tissues, it develops a cytoplasmic extension, the pollen tube, which is one of the fastest growing cells in nature. Pollen is completely “ready-to-go”, but despite this seemingly simple reaction, very complex interactions take place with the female tissues. In higher animals, genetic mechanisms for sex determination establish striking develop- mental differences between males and females. In contrast, most higher plant species develop both male and female structures within the same flower, allowing self-fertilization. Outcrossing is ensured by self-incompatibility mechanisms, which evolved under precise genetic control, control- ling self-recognition and cell-to-cell interaction. Equally important is pollen selection along the female tissues, where interactions between different cell types with inherent signalling properties correspond to check-points to ensure fertilization. Last but not least, pollen-pistil interaction occurs in a way that enables the correct targeting of the pollen tubes to the receptive ovules. In this review, we cover the basic mechanisms underlying sexual plant reproduction, from the structural and cellular determinants, to the most recent genetic advances. KEY WORDS: pollen tube, sexual plant reproduction, pollen-stigma interaction, fertilization, cell polarity Introduction style and into the ovule (Amici, 1824, 1830, 1847) (Fig. 1 C,D). The contact between the tip of the pollen tube and the egg cell remained The mechanisms of pollination and fertilization were guessed unanswered until Strasburger proved that the pollen tube does not from the practical utilization of agriculture and seed production stay intact, but its apical tip disintegrates upon contact with the more than 5000 years ago, when Assyrian priests had rituals of embryo sac and that one of its “nuclei fuses with the nucleus of the dusting pollen from the male date palms on the inflorescences of egg” (Maheshwari, 1950; Cresti and Linskens, 1999). It was left for female trees (Fig. 1 A,B). The Greek philosopher, Theophrastus Darwin (1862) to prove conclusively the importance of pollination (300 B.C) has also inferred on the union of the two sexes for the in perpetuation and vigor maintenance of plants. Double fertiliza- production of seed in plants. However, the idea was not general- tion was afterwards simultaneously described by Sergius Nawashin ized until 1682, when the botanist Nehemias Grew stated that and Leon Guignard in the dawn of the 19th century and nowadays pollen must reach the stigma to insure the development of seeds. is believed to be a ubiquitous phenomenon (Friedman, 2001). In 1694 Rudolph Jacob Camerarius published «De sexu plantarum epistole», in which he stated that based upon his experiments The male gametophyte in action - the pollen tube there are two different parts of the flower, the stamens and the pistil and that they must work together to produce ripe seed. After After pollination, pollen is somehow deposited on the stigma Brown’s (1833) detailed descriptions, it would be Amici (1824, surface, where it germinates by re-hydration and development of 1830), to firstly observe pollen tubes and its passage down the a fast-growing cytoplasmic extension, the pollen tube. Besides *Address correspondence to: Dr. José A. Feijó. Instituto Gulbenkian de Ciência, PT-2780-156, Oeiras, Portugal. Fax: +351-21-440-7970. email: [email protected] 0214-6282/2005/$25.00 © UBC Press Printed in Spain www.intjdevbiol.com 616 L.C. Boavida et al. A B C D Fig. 1. Historical perspective of the discovery of sexual reproduction in plants. (A,B) Ancient representations of the first historical evidence that pollen had to be brushed on the stigma surface as a means to assure seed production. As long as 5000 B.C. both Assyrian priests (B) and Egyptian gods were pictured on ceremonial fertilization of date palms. (C,D) One landmark of the scientific discovery of the basis of sexual plant reproduction was the precise definition of the pollen tube role in the discharge of the male component made by Giovanni Battista Amici. (C) shows a wax model by L. Calamai and E. Tortori under the supervision of Amici, illustrating a sagital section of the stigma and style of Cucurbita pepo showing the germination and progression of the pollen tube (Amici 1824)(original models are in the Botanical Museum of the University of Firenze, Italy). Amici later moved to explore the especially favourable structural features of orchids. (D) is reproduced from the original paper from Amici (1847) and shows camera lucida drawings of Orchis morio (1 to 8, 14, 15), Orchis mascula (9), Orchis abortiva (nowadays Limodorum abortivum )(10-12) and Orchis maculata (nowadays Dactylorhiza maculata )(13). 1 shows the pollen tubes (“boyaux”) reaching the ovaries and b - the transmitting tissue. It is remarkable how the author, using the optics and magnification of those days could so clearly distinguish the differences of the pollen tubes and the transmitting tissue cells. 2. Post- fertilized ovule. 3. Next stage of ovule maturation, illustrating the growth of the internal integument. 4. Next stage, with inversion and definition of the mycropyle. 5-8. Fecundation and beginning of embryogenesis. Amici could correctly describe the callose plugs and the discharge in one of the synergids. It took a lot of time and much more sophisticated techniques to see these details with such precision. 9 to 13. Fertilization in other species. 14 and 15, Double embryo formation in Orchis morio. How plants make a zygote 617 their essential role for fertilization, pollen tubes are an excellent switch from vegetative to reproductive actin isoforms, which are model system to study the cellular processes involved in polarity believed to fulfill unique functions during pollen development and tip-growth, e.g. cytoskeleton organization, formation and trans- (Kandasamy et al., 1999). port of secretory vesicles, endo- and exocytosis, polarized ion In recent years proteins that influence cell shape by regulating gradients and fluxes, periodic behavior and re-orientation of growth the organization and polymerization of cytoskeletal filaments were (reviewed by Mascarenhas, 1993; Feijó et al., 1995, 2001, 2004; described by different groups. Several actin binding proteins such Cheung, 1996b; Hepler et al., 2001). Cytological and molecular as profilin, villin and ADF/cofilin act as regulators of actin cytoskel- studies of these processes are regarded as important both for eton assembly and have been shown to play active roles in the fundamental studies of fertility control and reproduction in plants control of the differential distribution of organelles and vesicles and and also to elucidate the basic mechanisms in cellular polarization. in proper growth of the pollen tube (Kim et al., 1993; Staiger et al., With limitations some of these could provide templates to explain 1993; Lopez et al., 1996; Vidali and Hepler, 1997; Vidali et al., other tip-growing cells such as fungal hyphae, root hairs or certain 1999; Smertenko et al., 2001; Allwood et al., 2002; Chen et al., specialized animal cells. 2002; Hussey et al., 2002a; McKenna et al., 2004). Pollen tubes elongate dramatically (up to many centimeters) Contrasting with our current knowledge about actin filaments, through the female tissues to deliver the sperm cells into the the role microtubules play in pollen development and tube growth embryo sac. This growth occurs without further differentiation or remains unclear. Microtubules are made up of tubulin and are division and is restricted to the apical zone, thus depending on the known to interact with microtubule associated proteins secretion of membrane and cell wall materials at the pollen tube tip (MAP)(Hussey et al., 2002b). Microtubules were proposed to be (Fig. 2A) (Picton and Steer, 1981, 1983; Lancelle and Hepler, involved in the regulation of pollen tube morphogenesis and in the 1992; Roy et al., 1998; Parton et al., 2001). Pollen tubes can be maintenance of internal cytoplasmic organization, but not in active considered one of the fastest growing cells in nature, with rates of pollen tube growth (Joos et al., 1994; Derksen et al., 1995). They up to 4 µm.sec-1. also have been shown to regulate the position of the male germ unit Pollen tubes are intrinsically polarized cells, a feature reflected (Sorri et al., 1996; Rodriguez-Garcia et al., 2003). Microtubule